A solid catalyst containing as a component molybdenum oxide and a copper compound are often used as a catalyst and as an auxiliary agent, respectively, in production of acrylic acid. Liquid waste resulting from an acrylic acid production process contains the above metals.
Examples of a method for removing a metal from the liquid waste resulting from an acrylic acid production process generally include the following methods: (1) a method for removing a metal by adjusting a pH of the liquid waste, adding a precipitating agent, subsequently adding a coagulant, and separating the settled solid content using a filter press or a centrifuge; and (2) a method for removing a metal by further combining an adsorbent or an ion-exchange resin.
On the other hand, the liquid waste resulting from an acrylic acid production process contains various organic substances such as acetic acid and aldehydes. Thus, a method of wet combustion or wet oxidation is employed to convert the organic substances into harmless carbon dioxide gas, water, and the like. However, even if the organic substances are detoxified, the metal in the liquid waste remains intact in wastewater of the treated liquid.
The above conventional method for removing a metal from the liquid waste is not a technique intended for wastewater formed by subjecting the liquid waste to wet combustion treatment or wet oxidation treatment. Thus, the above method had problems when applied to the wastewater in that metal removal efficiency decreases depending on specific properties of the wastewater, and more specifically, in that amounts of agents used and concentrations of the remaining metals increase further compared to those of usual water.
Meanwhile, the “wet combustion” refers to a treatment method for liquid waste suppressing generation of NOx or SOx by subjecting the liquid waste to combustion treatment while collecting gas generated through combustion using an aqueous alkaline solution. The “wet oxidation” refers to another treatment method for liquid waste subjecting the liquid waste to oxidation treatment by supplying the liquid waste to be treated to water under high temperature and high pressure and further supplying oxygen thereto.
However, liquid waste containing acidic substances such as acrylic acid-containing liquid waste is generally neutralized with an alkali in advance and then supplied to a treatment equipment.